1.0 Field of the Invention
The present invention relates to a system, and a method of operation thereof, for measuring strain to which a structure having predetermined dimensions is subjected and, more particularly, to a system, and a method of operation thereof, for measuring strain using a technique for detecting the optical path difference (OPD) between first and second arms respectively of a sensing interferometer using a reading interferometer that is capable of being interrogated by changing the wavelength of the addressing light so as to change its optical path length. The reading interferometer can also be used for a variety of other low-coherence applications, in which the "sensing interferometer" can be used to measure strain, temperature or even be non fiber-optic, such as the cornea of a human eye. In the case of the human cornea, the reading interferometer with the wavelength based interrogation approach, described in this invention, can be used to measure the shape of the lens.
2.0 Description of the Prior Art
Strain gauges that detect the strain that stress produces in a body or structure are known. Strain gauges may consist of one or more fiber optic cables mated to the surface of the structure under test. In such arrangements, as the surface becomes strained, the optic fiber cable stretches, undergoing a change in length that is proportional to the change in strain. One such strain gauge system may be a long gauge sensor system which is attractive because its sensors can measure the average strain over long structures and which find usage in structural monitoring and damage assessment related to the effects of strain. Long gauge sensors have been proposed using low coherent differential interferometry techniques to sense strain over several meters of optical fiber. Two such proposals are described in the following two technical articles, one of Fan, N. Y., S. Huang and R. M. Measures, entitled "Localized Long Gauge Fiber Optic Strain Sensors," published in Smart Materials and Structures, April, 1998, and the other of Inaudi, D., A. Elamari, L. Pflug, N. Gisin, J. Breguet, S. Vurpillot, entitled "Low-Coherence Deformation Sensors for the Monitoring of Civil-Engineering Structures," published in Sensors and Actuators A, 1994, 44, pp. 125-130, and both of which technical articles are herein incorporated by reference. A more general reference on low coherence techniques is: "Recent Program in Fiber Optic Low Coherence Interferometer" of Y. J. Rae and O. A. Jackson published in Meas. Sci. Techn. (1998) pp. 981-999, and herein incorporated by reference.
The primary sensors described in the first two technical articles above require the use of mechanical activation to modify the path length of the interrogating interferometer, either by stretching the optical fiber, as described in the technical article of N. Y. Fan et al, or by moving a mirror, as described in the technical article of D. Inaudi et al. The technical article of N. Y. Fan et al also reports a technique which eliminates the need for mechanical activation based on the use of a tuneable laser and a fixed Fabry-Perot cavity. Although the proposal of N. Y. Fan et al to eliminate the mechanical activation is worthwhile, it is further desired that a system, and a method of operation thereof, be provided that not only eliminates mechanical activation of a sensor, but also improves the speed of response for detecting and decoding the sensed strain.